The present invention relates to an anisotropic conductive paste and a method of electrically connecting complicated electric circuits or forming an electric connection circuit of a liquid crystal display board with a semiconductor IC and/or an IC-loaded circuit board.
In recent years, as various devices including a personal computer are miniaturized and thinned, it has been increasingly difficult to connect very fine electric circuits themselves. In particular, a work to instill a semiconductor IC and IC-loaded circuit board on a narrow place becomes increasing. A means for electrically connecting them at a time used to rely on an advance in a soldering technique in the past. However, the existing situation is that in recent years, the connection is carried out by using an anisotropic conductive film.
However, in using an anisotropic conductive film, a work for peeling releasing paper and a punching work for molding are required. In order to save such works, it is essential to install an automatic peeling machine, an automatic punching machine and an automatic adhesion machine in combination, which are expensive, and there are the problems that a risk of an equipment investment is involved and a lot of wastes of an expensive anisotropic conductive film is produced.
On the other hand, anisotropic conductive pastes which are liquid at room temperature are proposed as those producing fewer wastes in the above field. Known are, for example, an anisotropic conductive paste prepared by providing an insulating layer on a conductive particle surface and dispersing the particle in an insulating adhesive component and an anisotropic conductive paste composition prepared by dispersing a conductive rubber particle in an insulating adhesive.
An anisotropic conductive paste means a thermosetting type liquid composition of a single liquid type by which fine conductive circuits themselves are connected, whereby the paste makes it possible to electrically connect and fix the circuits on upper and lower boards.
In recent years, an anisotropic conductive paste is strongly required to meet severe use, and those which are free of a resin-leaking phenomenon and bleeding contamination that occurs in a production line involving quick heat application and which do not cause problems such as inferior conduction between electrodes in a bonding direction and inferior insulation characteristics in a direction perpendicular to the bonding direction tend to be required.
The existing situation is that strongly desired to be developed particularly in the above field is an anisotropic conductive paste which is excellent in a pre-bonding property and does not cause resin leaking even under such severe hot press bonding conditions for maintaining high productivity as, for example, lower than 200xc2x0 C., a pressure of 2 MPa/m2 and 20 to 30 seconds and which has a high adhesion reliability.
In light of the social background described above, a subject to be solved by the present invention is to provide an anisotropic conductive paste satisfying the following problems at the same time.
That is, the subject is to provide an anisotropic conductive paste which does not substantially cause resin leaking and bubble generation in hot press bonding and curing and is non-contaminative and which is easy in hot press bonding and curing at 30 to 200xc2x0 C. for short time and can achieve highly reliable electric connection accompanied with anisotropy. Further, the subject is to provide a method of electrically connecting complicated electric circuits or forming an electric connection circuit of a liquid crystal display board with a semiconductor IC and/or an IC-loaded circuit board by using the above anisotropic conductive paste.
Intensive investigations repeated by the present inventors in order to solve the problems described above have resulted in finding that the subjects described above can be achieved by employing an anisotropic conductive paste that comprises an epoxy resin as a principal component and contains each specific amount of a rubber-like polymer fine particle, a thermally active potential curing agent for epoxy resin and a high softening point-polymer fine particle, a conductive particle having a specific size and if necessary, an inorganic filler, a silane coupling agent, a solvent which is compatible with the epoxy resin and has a boiling point falling in a range of 150 to 220xc2x0 C. and which is inactive to an epoxy group, and other additives, and thus the present invention has been completed.
That is, the anisotropic conductive paste of the present invention comprises the following items [1] to [11]:
[1] An anisotropic conductive paste comprising conductive fine particles and an epoxy resin composition, wherein
(a) an aqueous solution obtained by sufficiently admixing the above anisotropic conductive paste with the same mass of purified water has an ionic conductivity of 1 mS/m or less and
(b) a coated material obtained by coating the above anisotropic conductive paste in a thickness of 50 xcexcm and subjecting it to heat treatment at 80 to 100xc2x0 C. for 20 minutes has a viscosity of 50 to 10000 Pa.s at 80 to 100xc2x0 C. by an E type viscometer,
(c) the cured matter of the above anisotropic conductive paste has a linear expansion coefficient of 10xc3x9710xe2x88x925 mm/mm/xc2x0 C. or less at 0 to 100xc2x0 C., which is determined by means of a thermomechanical analyzer (TMA),
(d) the cured matter of the above anisotropic conductive paste has a heat deformation temperature Tg of 100xc2x0 C. or higher, which is determined by means of a thermomechanical analyzer (TMA),
(e) the cured matter of the above anisotropic conductive paste has a water absorption coefficient of 2 mass % or less, and
(f) the cured matter of the above anisotropic conductive paste has a specific resistance of 1xc3x97109xcexa9.cm or more.
[2] The anisotropic conductive paste as described in the above item [1], wherein the above epoxy resin composition comprises:
(1) 30 to 93 mass % of an epoxy resin having 1.2 or more epoxy groups on an average in a molecule,
(2) 1 to 15 mass % of a rubber-like polymer fine particle having a softening point of 0xc2x0 C. or lower and a primary particle diameter of 5 xcexcm or less,
(3) 5 to 60 mass % of a thermally active potential curing agent for epoxy resin, and
(4) 1 to 20 mass % of a high softening point-polymer fine particle having a softening point of 50xc2x0 C. or higher and a primary particle diameter of 2 xcexcm or less, and wherein the above anisotropic conductive paste comprises; 93 to 97 volume % of the epoxy resin composition comprising the components (1) to (4) described above, and
(5) 3 to 7 volume % of a conductive particle having an average particle diameter of 5 to 15 xcexcm, a maximum particle diameter of 20 xcexcm or less and a minimum particle diameter of 0.1 xcexcm or more.
[3] The anisotropic conductive paste as described in the above item [2], further comprising 1 to 10 mass % of an inorganic filler.
[4] The anisotropic conductive paste as described in the above item [2] or [3], further comprising 0.1 to 5 mass % of a silane coupling agent.
[5] The anisotropic conductive paste as described in any of the above items [2] to [4], wherein a maximum exothermic peak temperature determined from a thermogram obtained by differential scanning calorimetry (DSC) in which 10 mg of the above anisotropic conductive paste is heated at a constant rate of 5xc2x0 C./minute in an inert gas atmosphere is 80 to 180xc2x0 C.
[6] The anisotropic conductive paste as described in any of the above items [2] to [5], comprising a single liquid type epoxy resin composition, wherein an exothermic initiation temperature determined from a thermogram obtained by differential scanning calorimetry (DSC) in which 10 mg of the above anisotropic conductive paste is heated at a constant rate of 5xc2x0 C./minute in an inert gas atmosphere is 30 to 130xc2x0 C.
[7] The anisotropic conductive paste as described in any of the above items [4] to [6], wherein the epoxy resin (1) described above is an epoxy resin having 1.7 or more epoxy groups on an average in a molecule and has a polystyrene-reduced number average molecular weight of 7000 or less which is determined by gel permeation chromatography.
[8] The anisotropic conductive paste as described in any of the above items [4] to [7], wherein the components (2), (4) and (5) are present in the state that they are dispersed in the epoxy resin in the form of particles respectively.
[9] The anisotropic conductive paste as described in any of the above items [4] to [8], wherein the fine particle (4) is a high softening point-polymer fine particle which comprises a poly(meth)acrylate having a micro cross-linking structure as a main component and has a softening point of 60 to 150xc2x0 C. and a primary particle diameter falling in a range of 0.01 to 2 xcexcm, in which an epoxy group is introduced into a polymer component in a proportion of 0.1 to 5 mass %.
[10] The anisotropic conductive paste as described in any of the above items [4] to [9], wherein the conductive particle (5) comprises a core of an organic polymer and a coated layer of at least one metal selected from gold, silver, copper, nickel or an alloy thereof.
[11] A method of forming an electric connection circuit of a liquid crystal display board with a semiconductor IC and/or an IC-loaded circuit board by dispenser-coating the anisotropic conductive paste as described in any of the above items [2] to [10] on a part of the liquid crystal display board to be electrically connected with the semiconductor IC and/or the IC-loaded circuit board and press bonding them by means of a normal heat press at a temperature lower than 250xc2x0 C. to adhere and fix them making use of a thermal curing reaction of the above anisotropic conductive paste to thereby make vertically conductive and form an electric conductive circuit.